Gait disorders are a common problem in the aging population. Etiologies include neurological problems from neuropathies to neurodegenerative disorders, to strokes that affect locomotor circuitries. These circuitries are poorly understood, but the impact of the problem in terms of quality of life, and need for institutionalization is enormous. Locomotion is a complex function, requiring control of initiation of movement, speed, and rhythm. These functions are mediated via spinal central pattern generators (CPG), which are steered by sensory and supraspinal input. A major source of supraspinal input is from reticulospinal neurons in the medial medulla, in the lower brainstem. This region in turn receives input from mid- and forebrain locomotor regions from which locomotion is controlled. Inhibitory systems are crucial for locomotor control at spinal levels, but roles of inhibitory neurons could not be dissected in the medulla due to contributions from admixed serotonergic/glutamatergic reticulospinal neurons. Novel techniques now allow us to selectively study the functions and connectivity of inhibitory reticulospinal neurons. In our pilot studies we focally deleted the vesicular GABA transporter (vgat) from subregions in the medial medulla in conditional knockout mice and reconstructed connections of these regions to the spinal cord. Based upon our results we hypothesize that: 1) a dorsal inhibitory system is involved in initiation of movement. When this system is excited by locomotor regions, movement is initiated via disinhibition of spinal neurons. 2) a ventral inhibitory system regulates speed via projections to motoneurons and interneurons that modulate sensory feedback. We will test these hypotheses rigorously using Designer Receptors Exclusively Activated by Designer Drugs (DREADD) technology. This novel technology makes use of mutated receptors which can be built into selected groups of neurons. These receptors can then be selectively activated by administering an otherwise pharmacologically inert designer drug. This allows to reversibly inhibit or stimulate neurons, depending on the type of mutated receptor that was built in. This technology is suitable to functionally and anatomically dissect complex circuitries, and the overall approach has potential for applications in human disease. In Aim 1, we will assess the functions of inhibitory neurons in the medulla using DREADD technology. In Aim 2, we will selectively visualize the pathways from the various groups of inhibitory neurons in the medial medulla to the spinal cord, and will characterize classes of spinal neurons that are targeted by these systems. In Aim 3, we will use these same techniques to assess the nature and density of connections from putative locomotor regions in the fore- and midbrain to these inhibitory medullary systems. Our results will change current paradigms of locomotor control, will help understand how dysfunction of locomotor systems alters gait in neurological disorders, and may lead to new treatment options.